1. Field of the Invention
The present invention generally relates to a system and method for registering data using an anatomically referenced system of markers. Embodiments of the invention relate to a bite plate comprising fiducial markers.
2. Description of Related Art
Since the discovery of X-rays in 1895, film has been the primary medium for capturing, displaying, and storing radiographic images. It is a technology that practitioners are the most familiar and comfortable with in terms of technique and interpretation. Digital radiography is the latest advancement in dental imaging and is slowly being adopted by the dental profession. Digital imaging incorporates computer technology in the capture, display, enhancement, and storage of direct radiographic images. Digital imaging offers some distinct advantages over film, but like any emerging technology, it presents new and different challenges for the practitioner to overcome.
Film-based imaging consists of X-ray interaction with electrons in the film emulsion, production of a latent image, and chemical processing that transforms the latent image into a visible one. As such, radiographic film provides a medium for recording, displaying, and storing diagnostic information. Film-based images are described as analog images. Analog images are characterized by continuous shades of gray from one area to the next between the extremes of black and white. Each shade of gray has an optical density (darkness) related to the amount of light that can pass through the image at a specific site. Film displays higher resolution than digital receptors with a resolving power of about 16 lp/mm. However, film is a relatively inefficient radiation detector and, thus, requires relatively high radiation exposure. The use of rectangular collimation and the highest speed film are methods that reduce radiation exposure, but these techniques are not practiced commonly in private dental offices. Chemicals are needed to process the image and are often the source of errors and retakes. The final result is a fixed image that is difficult to manipulate once captured.
Digital imaging is the result of X-ray interaction with electrons in electronic sensor pixels (picture elements), conversion of analog data to digital data, computer processing, and display of the visible image on a computer screen. Data acquired by the sensor is communicated to the computer in analog form. Computers operate on the binary number system in which two digits (0 and 1) are used to represent data. These two characters are called bits (binary digit), and they form words eight or more bits in length called bytes. The total number of possible bytes for 8-bit language is 28=256. The analog-to-digital converter transforms analog data into numerical data based on the binary number system. The voltage of the output signal is measured and assigned a number from 0 (black) to 255 (white) according to the intensity of the voltage. These numerical assignments translate into 256 shades of gray. The human eye is able to detect approximately 32 gray levels. Some digital systems sample the raw data at a resolution of more than 256 gray values such as 10 bit or 12 bit values. The large number of gray values is reduced to 256 shades of gray with the advantage of controlling under or overexposed images.
Direct digital imaging systems produce a dynamic image that permits immediate display, image enhancement, storage, retrieval, and transmission of the image. Digital sensors are more sensitive than film and require significantly lower radiation exposure. Dynamic range or latitude is the range of exposures that will produce images within the useful density range. This corresponds to the straight-line portion of the Hurter and Driffield (H & D) curve or the characteristic curve. This curve demonstrates the relationship between exposure (number of X-rays) and optical density (darkness) of an image receptor. The scale of useful densities ranges from 0.6 (low density—light) to 3.0 (high density—dark). Beyond these parameters, the image is not diagnostic. Typically, the H & D curve for film has a stretched letter S appearance with the top curve known as the shoulder and the bottom curve the toe. Exposure changes in the shoulder (high exposure) and toe (low exposure) have little affect on density, but small changes in the straight-line portion between them significantly affect density. The more vertical the straight-line portion of the curve is, the smaller the range and the narrower the film latitude. In comparison, the dynamic range of charged coupled devices (CCDs) is linear with no shoulder or toe and is much wider than film.
For years users (e.g., dentists, surgeons, doctors) have dealt with the problem of no quantitative measures to determine the success of a particular treatment. For example, when evaluating bone height, changes can be masked by disparities in projection geometry. Digital subtraction radiography is a technique that allows quantitative determination in changes in radiographs. The premise is quite simple. A radiographic image is generated before a particular treatment is performed. At some time after the treatment, another image is generated. The two images are digitized and compared on a pixel-by-pixel basis. The resultant image shows only the changes that have occurred and “subtracts” those components of the image that are unchanged. The magnitude of the changes can then be measured by evaluating the histogram (graphic depiction of the distribution of gray levels) of the resultant image. If the exact projection geometry and receptor placement are not recreated, the changes in the subtracted image will demonstrate the effects of misregistration rather than the effects of a therapeutic intervention. Direct digital imaging has been a great help in the quest to take the technique of digital subtraction radiography out of the laboratory setting and actually use it clinically. Now that consistent file sizes can be achieved, the attention is being directed towards methods for recreating image receptor placement and projection geometry so dentistry can start to provide quantitative data about treatment outcomes.
One problem with digital subtraction radiography is acquiring images which are comparable. Comparable images include images, which are of essentially the same space, or area allowing registered data from separate images of the same space to be compared (e.g., subtracted). Currently acquiring comparable images is accomplished by repeatedly positioning a patient in a particular location and orientation relative to the medical apparatus on a number of separate occasions. For example, a patient may return on multiple days for radiation therapy (radiotherapy) in which a beam of radiation is directed toward a particular feature in the body (a “target”) such as a cancerous tumor. One approach to targeting the same feature at each session is, at each session, to first restrain the patient relative to the apparatus and then to determine the location of the target relative to the apparatus, for example, using the locations of fiducial markers on a patient. This same approach is also used in acquiring digital images of a patient using for example a CT scanner. In one approach, a “bite plate” with trackable markers is used to determine the position of the patient relative to a medical apparatus using a remote sensing system. The medical apparatus is adjusted according to the sensed position of the patient: Another approach to targeting the same feature at each session is to restrain the patient in precisely the same position relative to the apparatus at each session. In one such approach, a complex, cumbersome, and often-painful positioning device, such as a stereotactic head frame, is fixed to a patient prior to scanning. The device is left in place after scanning to later position or register the patient in the medical apparatus. In another approach to repeatable positioning, a molded synthetic cast of a patient's head is made, and split in half to allow removal and subsequent re-attachment to the head. A stereotactic frame is attached to the mold, thereby allowing repeatable positioning of the stereotactic frame. Features in the body are then targeted relative to the stereotactic frame. Current methods have several associated disadvantages. For example, current methods may increase a patient's discomfort by increasing the length of time required to perform a CT scan or some other procedure. Current methods typically employ large and cumbersome systems coupled to a patient. Systems may be temporarily positionable in a portion of a patient. Systems may be temporarily coupled to a patient resulting in further discomfort. The current invention is designed to overcome disadvantages and shortcomings of current methods and systems.